1. Field of the Invention
The invention concerns the field of mobile telecommunication networks or mobile radio networks and more particularly networks using TDMA (time division multiple access) digital transmission.
2. Description of the Prior Art
These networks are made up of cells each comprising a base transceiver station responsible for providing communication with mobile stations in the cell. Various standards have already been drawn up for terrestrial telecommunication systems with mobile terminals, in particular the GSM, DCS 1800 and PCS standards. Under the GSM standard, each base transceiver station (BTS) transmits signaling messages in a cell on a broadcast control channel (BCCH). The broadcast control channel also identifies the cell. A random access channel (RACH) is used for signaling messages from the mobile stations. The mobile stations use this channel during a call initialization phase to send either a request (uplink call) or an acknowledgement message (downlink call).
The invention is more particularly concerned with cells with two layers. These cells have a macrocell layer and a microcell layer. In such cells, the macrocell layer is used to manage the mobile stations at cell level. The microcell layer is used to manage the mobile stations at micro-BTS level. Each micro-BTS radiates at a low power in the microcell that it defines, which typically has a size of a few hundred meters, compared to a typical size in the order of one kilometer for a cell or macrocell.
This design and this structure of the cells increases traffic capacity or alleviates transmission problems encountered in urban environments. However, the resulting improvement is greater if the mobile stations are managed in the microcell layer. An objective in cells with distributed antennas is therefore to assure as much transmission to and from the mobile stations in the microcell layer as possible. It is nevertheless still possible to transmit to and from the mobile stations in the macrocell layer. To handover from the microcell layer to the macrocell layer, it has been proposed to estimate the speed of the mobile stations and to use that speed in the decision whether to handover from the microcell to the macrocell. In this case, the mobile stations which are stationary or moving at low speed are managed in the microcell layer and fast-moving mobile stations are managed in the macrocell layer. This solution is described in ETSI recommendation TS GSM 05.22 version 4.00, for example. This recommendation proposes to evaluate the speed by the time spent in the cell.
French patent application 97 11123 filed Sep. 8, 1997 by the assignees of the inventors, whose title in translation is xe2x80x9cBase transceiver station controlling calls used in a cell of a digital cellular mobile radio network, and corresponding control methodxe2x80x9d, which had not been published by the date of filing of this application, describes a distributed radio coverage system. The above application proposes that each cell comprise a plurality of relay stations. Also, calls to a mobile station of the cell are allocated a set of at least two relay stations. The relay stations of the same cell use the same BCCH and RACH.
French patent application 97 11125 filed Sep. 8, 1997 by the assignees of the inventors, and whose title in translation is xe2x80x9cMethod of identifying a mobile station in a cell of a digital cellular mobile radio networkxe2x80x9d, which also had not been published at the date of this application, describes a distributed radio coverage system. The above application proposes that each cell comprise a plurality of relay stations. It proposes to select at least one of the relay stations of a cell in accordance with the quality of a call from a mobile station, in order to determine a position of this mobile station in the cell. The relay stations of the same cell use the same BCCH and RACH.
The invention proposes a solution to the problem of redirecting traffic from the microcell layer to the macrocell layer in distributed cells. It maximizes traffic efficiency and use of microcell resources and provides a good grade of service (GoS).
To be more precise, the invention proposes a method of communicating with a mobile station in a cell of a cellular telecommunication network having a microcell layer with a plurality of microcells each comprising at least one relay station and a macrocell layer with a macrocell covering a plurality of said microcells, wherein a call in the microcell layer is effected between the mobile station and a set of relay stations, the number of which is a cardinal number at least equal to two, allocated to the call, the relay stations of the set allocated to a call can change during the call, and the call is handed over from the microcell layer to the macrocell layer when it is not possible to allocate to the call a number of relay stations corresponding to said cardinal number.
Said cardinal number is advantageously 2.
In one embodiment, the macrocell comprises means for managing the equivalent of a number Nmacro of transmitter-receivers and said number is chosen so that the grade of service is less than or equal to 2%.
The invention also concerns a base transceiver station for a cell of a telecommunication network having a microcell layer with a plurality of microcells each comprising at least one relay station and a macrocell layer with one macrocell covering a plurality of said microcells, means for allocating a set of relay stations to a call in the microcell layer, the number of which is a cardinal number at least equal to two, allocated to the call, said allocation means allocating different relay stations to a mobile station in movement, and the base transceiver station further including means for handing over a call from the microcell layer to the macrocell layer when it is not possible to allocate to the call a number of relay stations corresponding to said cardinal number.
The cardinal number is preferably 2.
In one embodiment, the layer comprises means for managing the equivalent of a number Nmacro of transmitter-receivers and in that said number is chosen so that the grade of service is less than or equal to 2%.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention which is given by way of example and with reference to the accompanying drawings.